Google's Matt Cutts discusses loss of PageRank from 301 redirects in the latest Webmaster Help video. Specifically, he answers the following user-submitted question:
Roughly what percentage of PageRank is lost through a 301 redirect?
After providing some history about the context of this question, Cutts says, "I sent an email to the team who is in charge of this, and of course the implementation of this can very over time, but this has been roughly the same for quite a while. The amount of PageRank that dissipates through a 301 is currently identical to the amount of PageRank that dissipates through a link. So they are utterly the same in terms of the amount of PageRank that dissipates going through a 301 versus through a link. So that doesn't mean use a 301. It doesn't mean use a link. It means use whatever is best for your purposes because you don't get to hoard or conserve any more PageRank if you use a 301, and likewise it doesn't hurt you if you use a 301."
He is careful to point out that this could change in the future.
"That's the current implementation," he says. "We don't promise it will be that way for all time and eternity, but I don't see any reason why in particular it would change."